The resolution backed an Arab League plan calling on President Bashar Assad to cede power. In the Syrian city of Homs, shelling leaves 200 dead, activists say.

A United Nations resolution that Washington and its allies called the best chance to stop Syria from sliding into full-fledged civil war went down to defeat, dashing hopes for a political settlement as death tolls soar in the strategically situated Arab nation.

Security Council vetoes by superpowers Russia and China on Saturday doomed the measure, which condemned a Syrian crackdown on dissent and backed an Arab League plan calling on President Bashar Assad to cede power.

The collapse of the peace initiative came despite President Obama's strongest condemnation yet of what he called Assad's "killing machine."

"Assad must halt his campaign of killing and crimes against his own people now," Obama declared before the vote, reacting to accounts of an overnight shelling in the restive city of Homs that opposition activists said left at least 200 people dead. "He must step aside and allow a democratic transition to proceed immediately."

Instead, the veto by Russia and China threw a lifeline to Assad, whose family has ruled Syria for more than 40 years through deft manipulation of regional intrigues, an essential skill for a nation in a volatile region, bordering Israel, Iraq, Turkey, Lebanon and Jordan.

The vote would seem to signal a death knell for the Arab League's peace road map, hammered out during months of negotiation. There is no other pending peace plan.

That leaves the United States and its allies with few options for halting the bloodshed. The U.S. and Europe have already imposed a variety of economic sanctions on the Assad government. But observers say Assad is convinced his regime can survive the sanctions regimen, despite the country's steep economic downturn since protests began.

The violence has reached increasingly disturbing levels as more of Assad's opponents take up arms. After reports of the shelling Friday night in Homs, activists called it the bloodiest 24-hour period of an uprising now nearing its one-year anniversary, with one witness saying, "There were corpses everywhere, blown apart from the forces of the explosion." The Syrian government dismissed the shelling reports as a fabrication designed to sway diplomats at the U.N.

Syria rejected the Arab League blueprint as an infringement on its sovereignty, a stance backed by Russia and China, both wary of the prospect of Western-engineered leadership change just six months after Western airstrikes helped the Libyan opposition overthrow Moammar Kadafi.

The Arab League plan was freighted with geopolitical significance. Saudi Arabia and other Arab countries of the Persian Gulf, which supported the plan, are locked in a regional power struggle with Iran, a key ally of Syria. The Arab countries view the prospect of Assad's departure as a blow against Iran in its bid to wield wider influence in the Middle East.

But Assad still has support among many Syrians, especially Christians and other minorities who view his secular administration as a bulwark against Iraq-style sectarian warfare. Assad has blamed Islamic "terrorists" for his troubles.

The conflict in Syria has increasingly taken on the trappings of a civil war. The rebel Free Syrian Army says it has thousands of defectors and other volunteers under its command and is committed to armed struggle against the Assad regime. Rebels have been able to occupy towns and neighborhoods in recent weeks, at least until troops arrive with tanks to drive them out.

Meanwhile, Assad's military, while stretched thin, remains a formidable force. The army and intelligence services are mostly under the control of loyalists of the president's Alawite sect, an offshoot of Shiite Islam.

Some diplomats voiced the hope that a revised version of the U.N. resolution could be proposed at some point. But after two joint vetoes — the Russians and Chinese blocked an earlier resolution on Syria in October — there is no guarantee that a consensus can be reached on any wording about Syria.

The Arab League plan remains in place, at least theoretically. Arab diplomats could choose to send in new observer teams or try to bring Syria back to the negotiating table. But it is not clear whether cooperation would be forthcoming from Syria or the Arab countries, which pulled their observers after complaining that Assad's government was doing little to stop the violence.

The U.N. vote Saturday was a stinging rebuke for the United States and its allies, who had expressed hope as recently as late Friday that Russia might be persuaded to join the resolution, or at least abstain.

Diplomats had worked intensely for days to hammer out a compromise acceptable to Russia, removing threats of economic sanctions and an arms embargo against Syria and ruling out Libya-style U.N.-sanctioned international intervention. But it wasn't enough.